The invention relates to a method and an apparatus for indicating enciphering of data transmission between a mobile communication network and a mobile station in the mobile communication network.
In mobile networks, at least part of the data transmission is wireless communication using radio transmitters and receivers. The radio channel is a physically open resource available to anyone by means of suitable communication equipment. This involves security risks, such as eavesdropping or disclosure of privacy of location. In digital mobile networks, such as GSM networks, digital data transmission is used which is difficult to eavesdrop. Further, it is possible to use identification of the caller and enciphering in data transmission. For preventing eavesdropping in digital mobile networks, enciphering methods have been developed for enciphering the speech and data signals modified in digital form. Moreover, enciphering can be used in the transmission of other information via the radio channel, such as identification data on the mobile station (International Mobile Subscriber Identity, IMSI) and on the location (Location Area Identification, LAI). In the receiver, the enciphered signal is deciphered back to deciphered speech and data. A so-called encryption key and algorithm are advantageously known to the respective sending and receiving devices only, wherein given the effective encryption algorithms presently in use, decoding a coded signal to intelligible speech and data as well as into processing signals of the bit stream by force or illegally, i.e. without the correct encryption key and algorithm, is practically impossible.
The most common digital mobile networks are cellular networks. The base station subsystem (BSS) of the mobile network comprises base transceiver stations (BTS) and base station controllers (BSC). The mobile station (MS) communicates via the radio channel with a base station close to the respective location of the mobile station. The base station communicates with the base station controller. Data transmission between the base station and the base station controller takes place usually via a cable. One base station controller controls over a group of several base stations. The base station controller, in turn, communicates with a mobile services switching centre (MSC). Several mobile services switching centres, in turn, can communicate with each other as well as with a landline communication network centre (PSTN, ISDN). The information to be transmitted is usually divided into frames containing control information, speech and data converted into digital form, and error correction information. The frame structure can have several levels, wherein frames of a higher level are formed by arranging frames of a lower level. Enciphering can be used both with control information and with speech and data. Moreover, enciphering can be realized by using different encryption keys and algorithms at different frame levels. An example of a digital data transmission network is the GSM network, the standard of which contains definitions of the enciphering methods and algorithms to be used.
In the GSM network, making a mobile-originated call is conducted in a way that a GSM mobile station and the GSM system network give signals, i.e. transmit control and identification information required for making a call. In response to a request for a connection, the GSM mobile station is allotted a channel for signalling, if this is possible within the capacity of the GSM system network. On this channel, the GSM mobile station requests speech or data service from the GSM system network. On the side of the GSM system network, this request is transmitted to a mobile services switching centre (MSC), where the rights of the GSM subscriber are checked from a visitor location register (VLR).
Upon a mobile-terminated call e.g. from a subscription of a landline telephone network, the operator of the telephone network transmits e.g. the number of the receiving telephone to the mobile services switching centre. The mobile services switching centre finds out the rights of the GSM subscriber from the home location register (HLR) and from the visitor location register (VLR). After this, the GSM system network and the GSM mobile station transmit the control and identification information required for making a call.
Depending on the implementation and the configuration of the parameters, the visitor location register VLR can make a request via the mobile services switching centre to the GSM mobile station for exchange of identification information and start of enciphering. This request is made in a so-called cipher mode command message. It is, however, possible to make a call also without exchange of identification information and enciphering. In other words, the call is either enciphered or not enciphered depending on the network parameters set by the GSM system network operator.
In most common mobile communication networks currently available, however, enciphering is not optional to a user of the mobile communication network but usually an alternative function offered by the operator of the mobile network, wherein when current wireless data transmission equipment is used, the user has no certainty whether the data transmission is enciphered or not. Particularly when the mobile station is moving, the mobile station can be transferred from the area of one base station system to the area of another base station system, wherein the cipher mode of data transmission can be changed.